Photoelectric conversion device and manufacturing method thereof

ABSTRACT

A photoelectric conversion device having a new anti-reflection structure is provided. A photoelectric conversion device includes a first-conductivity-type crystalline semiconductor region that is provided over a conductive layer; a crystalline semiconductor region that is provided over the first-conductivity-type crystalline semiconductor region and has an uneven surface by including a plurality of whiskers including a crystalline semiconductor; and a second-conductivity-type crystalline semiconductor region that covers the uneven surface of the crystalline semiconductor region having the uneven surface, the second conductivity type being opposite to the first conductivity type. In the photoelectric conversion device, a concentration gradient of an impurity element imparting the first conductivity type is formed from the first-conductivity-type crystalline semiconductor region toward the crystalline semiconductor region having the uneven surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/157,570, filed Jun. 10, 2011, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2010-139993 on Jun. 18, 2010, both of which are incorporated byreference.

TECHNICAL FIELD

The present invention relates to a photoelectric conversion device and amethod for manufacturing the same.

BACKGROUND ART

Recently, a photoelectric conversion device, which is a power generationmeans that generates power without carbon dioxide emissions, hasattracted attention as a countermeasure against global warming. A solarcell for supplying residential power or the like, which generates powerfrom sunlight outdoors, is known as a typical example thereof. For sucha solar cell, a crystalline silicon solar cell using single crystalsilicon or polycrystalline silicon is mainly used.

An uneven structure is provided on a surface of a solar cell using asingle crystal silicon substrate or a polycrystalline silicon substratein order to reduce surface reflection. The uneven structure provided onthe surface of the silicon substrate is formed by etching the siliconsubstrate with an alkaline solution such as an aqueous sodium hydroxidesolution. Since the etching rate by the alkaline solution variesdepending on a crystal plane orientation of silicon, when a siliconsubstrate with a (100) plane is used for example, a pyramidal unevenstructure is formed.

Although the above uneven structure can reduce surface reflection of thesolar cell, the alkaline solution used for etching causes contaminationof the silicon substrate. In addition, since etching characteristicsconsiderably vary depending on the concentration or temperature of thealkaline solution, it is difficult to form the uneven structure on thesurface of the silicon substrate with high reproducibility. For thedifficulty, a combination method of a laser processing technique andchemical etching is disclosed (for example, see Patent Document 1).

On the other hand, in a solar cell whose photoelectric conversion layeris formed using a semiconductor thin film of silicon or the like, it isdifficult to form an uneven structure on a surface of the silicon thinfilm by above etching using an alkaline solution.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2003-258285

DISCLOSURE OF INVENTION

In any case, the method in which the silicon substrate itself is etchedto form the uneven structure on the surface of the silicon substrate isnot favorable because the method has a problem in controllability of theuneven shape and affects the characteristics of the solar cell. Inaddition, since the alkaline solution and a large amount of rinse waterare needed for etching of the silicon substrate and it is necessary topay attention to the contamination of the silicon substrate, the methodis also not favorable in terms of productivity.

Thus, an object of an embodiment of the present invention is to providea photoelectric conversion device having a novel anti-reflectionstructure.

One feature of an embodiment of the present invention is to form anuneven structure on a surface of a semiconductor by crystal growth ofthe same or different kind of semiconductor instead of forming ananti-reflection structure by etching a surface of a semiconductorsubstrate or a semiconductor film.

For example, by providing a semiconductor layer including a plurality ofprotrusions on a light incident plane side of a photoelectric conversiondevice, surface reflection can be considerably reduced. Such a structurecan be formed by a vapor deposition method; therefore, the contaminationof the semiconductor is not caused.

By a vapor deposition method, a semiconductor layer including aplurality of whiskers as an uneven structure can be grown, whereby ananti-reflection structure of the photoelectric conversion device can beformed.

One embodiment of the present invention is a photoelectric conversiondevice including a first-conductivity-type crystalline semiconductorregion that is provided over a conductive layer; a crystallinesemiconductor region that is provided over the first-conductivity-typecrystalline semiconductor region and has an uneven surface by includinga plurality of whiskers including a crystalline semiconductor; and asecond-conductivity-type crystalline semiconductor region that coversthe uneven surface of the crystalline semiconductor region having theuneven surface, the second conductivity type being opposite to the firstconductivity type. In the photoelectric conversion device, aconcentration gradient of an impurity element imparting the firstconductivity type is formed from the first-conductivity-type crystallinesemiconductor region toward the crystalline semiconductor region havingthe uneven surface.

One embodiment of the present invention is a photoelectric conversiondevice including a first-conductivity-type crystalline semiconductorregion, an intrinsic crystalline semiconductor region, and asecond-conductivity-type crystalline semiconductor region that arestacked over an electrode. The intrinsic crystalline semiconductorregion includes a flat crystalline semiconductor region, and a pluralityof whiskers that are provided over the flat crystalline semiconductorregion and include a crystalline semiconductor. That is, the intrinsiccrystalline semiconductor region includes the plurality of whiskers;thus, a surface of the second-conductivity-type crystallinesemiconductor region is uneven. In addition, an interface between theintrinsic crystalline semiconductor region and thesecond-conductivity-type crystalline semiconductor region is uneven. Inthe photoelectric conversion device, a concentration gradient of animpurity element imparting the first conductivity type is formed fromthe first-conductivity-type crystalline semiconductor region toward theintrinsic crystalline semiconductor region.

One embodiment of the present invention is a photoelectric conversiondevice including a first-conductivity-type crystalline semiconductorregion, an intrinsic crystalline semiconductor region, and asecond-conductivity-type crystalline semiconductor region that arestacked over an electrode. The first-conductivity-type crystallinesemiconductor region includes a flat crystalline semiconductor regionincluding an impurity element imparting the first conductivity type, anda plurality of whiskers that are provided over the flat crystallinesemiconductor region and include a crystalline semiconductor includingthe impurity element imparting the first conductivity type. That is, thefirst-conductivity-type crystalline semiconductor region includes theplurality of whiskers; thus, a surface of the second-conductivity-typecrystalline semiconductor region is uneven. In addition, an interfacebetween the first-conductivity-type crystalline semiconductor region andthe intrinsic crystalline semiconductor region is uneven. In thephotoelectric conversion device, a concentration gradient of theimpurity element imparting the first conductivity type is formed fromthe first-conductivity-type crystalline semiconductor region toward theintrinsic crystalline semiconductor region.

Note that in the above photoelectric conversion device, thefirst-conductivity-type crystalline semiconductor region is one of ann-type semiconductor region and a p-type semiconductor region, and thesecond-conductivity-type crystalline semiconductor region is the otherof the n-type semiconductor region and the p-type semiconductor region.

One embodiment of the present invention is a photoelectric conversiondevice including, in addition to the above structure, athird-conductivity-type semiconductor region, an intrinsic semiconductorregion, and a fourth-conductivity-type semiconductor region that arestacked over the second-conductivity-type crystalline semiconductorregion. Here, a surface of the second-conductivity-type crystallinesemiconductor region is uneven. Note that the band gap of the intrinsiccrystalline semiconductor region is different from the band gap of theintrinsic semiconductor region.

Note that in the above photoelectric conversion device, thefirst-conductivity-type crystalline semiconductor region and thethird-conductivity-type semiconductor region are one of an n-typesemiconductor region and a p-type semiconductor region, and thesecond-conductivity-type crystalline semiconductor region and thefourth-conductivity-type semiconductor region are the other of then-type semiconductor region and the p-type semiconductor region.

Directions of axes of the plurality of whiskers which are provided inthe first-conductivity-type crystalline semiconductor region or theintrinsic crystalline semiconductor region may be the direction normalto the electrode. Alternatively, the directions of axes of the pluralityof whiskers which are provided in the first-conductivity-typecrystalline semiconductor region or the intrinsic crystallinesemiconductor region may be varied.

The electrode includes a conductive layer. The conductive layer can beformed using a metal element which forms silicide by reacting withsilicon. Alternatively, the conductive layer can be formed with astacked layer structure including a layer which is formed using amaterial having high conductivity such as a metal element typified byplatinum, aluminum, or copper, and a layer which is formed using a metalelement which forms silicide by reacting with silicon.

The electrode may include a mixed layer covering the conductive layer.The mixed layer includes a metal element and silicon. The mixed layermay include silicon and a metal element which is included in theconductive layer. In the case where the conductive layer is formed usinga metal element which forms silicide by reacting with silicon, the mixedlayer may be formed of silicide.

In the photoelectric conversion device, the first-conductivity-typecrystalline semiconductor region or the intrinsic crystallinesemiconductor region includes a plurality of whiskers; thus, lightreflectance at the surface can be reduced. In addition, since thephotoelectric conversion layer absorbs light incident on thephotoelectric conversion layer owing to a light-trapping effect,characteristics of the photoelectric conversion device can be improved.

One embodiment of the present invention is a method for manufacturing aphotoelectric conversion device, including the steps of forming afirst-conductivity-type crystalline semiconductor region by a lowpressure chemical vapor deposition method (hereinafter, also referred toas a low pressure CVD method or an LPCVD method) using a deposition gascontaining silicon and a gas imparting the first conductivity type as asource gas over a conductive layer; forming an intrinsic crystallinesemiconductor region that includes a crystalline semiconductor regionand a plurality of whiskers including a crystalline semiconductor by alow pressure CVD method using a deposition gas containing silicon as asource gas over the first-conductivity-type crystalline semiconductorregion, and moving an impurity element imparting the first conductivitytype from the first-conductivity-type crystalline semiconductor regiontoward the intrinsic crystalline semiconductor region; and forming asecond-conductivity-type crystalline semiconductor region by a lowpressure CVD method using a deposition gas containing silicon and a gasimparting the second conductivity type as a source gas over theintrinsic crystalline semiconductor region.

One embodiment of the present invention is a method for manufacturing aphotoelectric conversion device, including the steps of forming afirst-conductivity-type crystalline semiconductor region that includes acrystalline semiconductor region and a plurality of whiskers including acrystalline semiconductor by a low pressure CVD method using adeposition gas containing silicon and a gas imparting the firstconductivity type as a source gas over a conductive layer; forming anintrinsic crystalline semiconductor region by a low pressure CVD methodusing a deposition gas containing silicon as a source gas over thefirst-conductivity-type crystalline semiconductor region, and moving animpurity element imparting the first conductivity type from thefirst-conductivity-type crystalline semiconductor region toward theintrinsic crystalline semiconductor region; and forming asecond-conductivity-type crystalline semiconductor region by a lowpressure CVD method using a deposition gas containing silicon and a gasimparting the second conductivity type as a source gas over theintrinsic crystalline semiconductor region.

Note that the low pressure CVD method is performed at a temperaturehigher than 550° C. In addition, silicon hydride, silicon fluoride, orsilicon chloride may be used for the deposition gas containing silicon.In addition, the gas imparting the first conductivity type is one ofdiborane and phosphine, and the gas imparting the second conductivitytype is the other of the diborane and the phosphine.

By a low pressure CVD method, the first-conductivity-type crystallinesemiconductor region which includes the plurality of whiskers or theintrinsic crystalline semiconductor region which includes the pluralityof whiskers can be formed over the conductive layer which is formedusing a metal element which forms silicide by reacting with silicon.

Note that in this specification, an “intrinsic semiconductor” refers tonot only a so-called intrinsic semiconductor in which the Fermi levellies in the middle of the band gap, but a semiconductor in which theconcentration of an impurity imparting p-type or n-type conductivity is1×10²⁰ cm⁻³ or lower and photoconductivity is 100 times or more as highas the dark conductivity. This intrinsic semiconductor may include animpurity element belonging to Group 13 or Group 15 of the periodictable. Accordingly, the problems can be solved even with the use of asemiconductor having n-type or p-type conductivity as well as the use ofthe intrinsic semiconductor, and thus another semiconductor having asimilar effect can be used. Such a substantially intrinsic semiconductoris included in an intrinsic semiconductor in this specification.

According to an embodiment of the present invention, the surface of thesecond-conductivity-type crystalline semiconductor region is uneven,whereby the characteristics of the photoelectric conversion device canbe improved. In other words, by providing a group of whiskers for aplane on a light incident side of the intrinsic crystallinesemiconductor region, surface reflection can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a cross-sectional view illustrating a photoelectric conversiondevice;

FIG. 2 is a cross-sectional view illustrating a photoelectric conversiondevice;

FIG. 3 is a cross-sectional view illustrating a photoelectric conversiondevice;

FIGS. 4A to 4C are cross-sectional views illustrating a method formanufacturing a photoelectric conversion device;

FIG. 5 is a cross-sectional view illustrating a photoelectric conversiondevice;

FIGS. 6A and 6B are cross-sectional views illustrating a photoelectricconversion device;

FIGS. 7A and 7B are cross-sectional views illustrating a photoelectricconversion device;

FIG. 8 is a cross-sectional view illustrating a photoelectric conversiondevice; and

FIG. 9 is a graph showing light regular reflectance.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments and an example of the present invention will bedescribed with reference to the drawings. Note that the invention is notlimited to the following description, and it will be readily appreciatedby those skilled in the art that various changes and modifications canbe made without departing from the spirit and scope of the invention.Thus, the present invention should not be construed as being limited tothe following description of the embodiments and example. In descriptionwith reference to the drawings, in some cases, the same referencenumerals are used in common for the same portions in different drawings.Further, in some cases, the same hatching patterns are applied tosimilar parts, and the similar parts are not necessarily designated byreference numerals.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, the scale of each structure is notnecessarily limited to that illustrated in the drawings.

Note that terms such as first, second, and third in this specificationare used in order to avoid confusion among components, and the terms donot limit the components numerically. Therefore, for example, the term“first” can be replaced with the term “second”, “third”, or the like asappropriate.

Embodiment 1

In this embodiment, a structure of a photoelectric conversion devicewhich is one embodiment of the present invention is described withreference to FIG. 1, FIG. 2, FIG. 3, and FIGS. 4A to 4C.

A photoelectric conversion device described in this embodiment includesa first-conductivity-type crystalline semiconductor region provided overa conductive layer, a crystalline semiconductor region which is providedover the first-conductivity-type crystalline semiconductor region andhas an uneven surface by including a plurality of whiskers including acrystalline semiconductor, and a second-conductivity-type crystallinesemiconductor region provided to cover the uneven surface of thecrystalline semiconductor region having the uneven surface. The secondconductivity type is opposite to the first conductivity type.

FIG. 1 is a photoelectric conversion device including a substrate 101,an electrode 103, a first-conductivity-type crystalline semiconductorregion 107, an intrinsic crystalline semiconductor region 109, asecond-conductivity-type crystalline semiconductor region 111, and aninsulating layer 113. The second conductivity type is opposite to thefirst conductivity type. The first-conductivity-type crystallinesemiconductor region 107, the intrinsic crystalline semiconductor region109, and the second-conductivity-type crystalline semiconductor region111 function as a photoelectric conversion layer 106. The crystallinesemiconductor region which has an uneven surface by including aplurality of whiskers including a crystalline semiconductor is formed inthe intrinsic crystalline semiconductor region 109. The insulating layer113 is formed over the second-conductivity-type crystallinesemiconductor region 111.

In this embodiment, an interface between the electrode 103 and thefirst-conductivity-type crystalline semiconductor region 107 is flat.The intrinsic crystalline semiconductor region 109 includes a flatportion and a plurality of whiskers (a group of whiskers). In otherwords, the interface between the electrode 103 and thefirst-conductivity-type crystalline semiconductor region 107 is flatwhile a surface of the second-conductivity-type crystallinesemiconductor region 111 is uneven. In addition, an interface betweenthe intrinsic crystalline semiconductor region 109 and thesecond-conductivity-type crystalline semiconductor region 111 is uneven.

In this embodiment, a p-type crystalline semiconductor layer and ann-type crystalline semiconductor layer are used as thefirst-conductivity-type crystalline semiconductor region 107 and thesecond-conductivity-type crystalline semiconductor region 111,respectively; however, the p-type conductivity and the n-typeconductivity may be interchanged with each other.

A crystalline silicon layer is used as the intrinsic crystallinesemiconductor region 109. Note that in this specification, an “intrinsicsemiconductor” refers to not only a so-called intrinsic semiconductor inwhich the Fermi level lies in the middle of the band gap, but also asemiconductor in which the concentration of an impurity imparting p-typeor n-type conductivity is less than or equal to 1×10²⁰ cm⁻³ and thephotoconductivity is 100 times or more as high as the dark conductivity.This intrinsic semiconductor may include an impurity element belongingto Group 13 or Group 15 of the periodic table.

As the substrate 101, a glass substrate typified by an aluminosilicateglass substrate, a barium borosilicate glass substrate, and analuminoborosilicate glass substrate, a sapphire substrate, a quartzsubstrate, or the like can be used. Alternatively, a substrate in whichan insulating film is formed over a metal substrate such as a stainlesssteel substrate may be used. In this embodiment, a glass substrate isused as the substrate 101.

Note that the electrode 103 may include only the conductive layer 104.Alternatively, the electrode 103 may include the conductive layer 104and a mixed layer 105 which is formed on a surface of the conductivelayer. Further alternatively, the electrode 103 may include only themixed layer 105.

The conductive layer 104 is formed using a metal element which formssilicide by reacting with silicon. Alternatively, the conductive layer104 may have a stacked layer structure which includes a layer formedusing a metal element having high conductivity typified by platinum,aluminum, copper, titanium, and an aluminum alloy to which an elementwhich improves heat resistance, such as silicon, titanium, neodymium,scandium, or molybdenum, is added on the substrate 101 side; and a layerformed using a metal element which forms silicide by reacting withsilicon on the first-conductivity-type crystalline semiconductor region107 side. Examples of the metal element which forms silicide by reactingwith silicon include zirconium, titanium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, cobalt, and nickel.

The mixed layer 105 may be formed using silicon and the metal elementwhich is included in the conductive layer 104. Note that in the casewhere the mixed layer 105 is formed using silicon and the metal elementincluded in the conductive layer 104, active species of a source gas aresupplied to a portion being deposited depending on heat conditions inthe formation of the first-conductivity-type crystalline semiconductorregion 107 by an LPCVD method, and thus silicon is diffused into theconductive layer 104 to form the mixed layer 105.

In the case where the conductive layer 104 is formed using a metalelement which forms silicide by reacting with silicon, silicideincluding the metal element is formed in the mixed layer 105. Thesilicide is typically one or more of zirconium silicide, titaniumsilicide, hafnium silicide, vanadium silicide, niobium silicide,tantalum silicide, chromium silicide, molybdenum silicide, cobaltsilicide, and nickel silicide. Alternatively, an alloy layer of siliconand a metal element which forms silicide is formed.

In the case where the mixed layer 105 is provided between the conductivelayer 104 and the first-conductivity-type crystalline semiconductorregion 107, resistance at an interface between the conductive layer 104and the first-conductivity-type crystalline semiconductor region 107 canbe further reduced; therefore, series resistance can be further reducedas compared to the case where the first-conductivity-type crystallinesemiconductor region 107 is directly stacked over the conductive layer104. In addition, the adhesiveness between the conductive layer 104 andthe first-conductivity-type crystalline semiconductor region 107 can beincreased. As a result, yield of the photoelectric conversion device canbe improved.

Note that the conductive layer 104 may have a foil shape, a plate shape,or a net shape. With such a shape, the conductive layer 104 can hold itsshape by itself, and the substrate 101 is therefore not essential. Forthis reason, cost can be reduced. In addition, when the conductive layer104 has a foil shape, a flexible photoelectric conversion device can bemanufactured.

The first-conductivity-type crystalline semiconductor region 107 istypically formed using a semiconductor to which an impurity elementimparting the first conductivity type is added. Silicon is suitable fora semiconductor material, in terms of productivity, a price, or thelike. When silicon is used as the semiconductor material, phosphorus orarsenic, which imparts n-type conductivity, or boron, which impartsp-type conductivity, is used as the impurity element imparting the firstconductivity type. Here, the first-conductivity-type crystallinesemiconductor region 107 is formed using a p-type crystallinesemiconductor.

Note that although the first conductivity type is p-type in thisembodiment, the first conductivity type may be n-type.

The intrinsic crystalline semiconductor region 109 includes acrystalline semiconductor region 109 a and a group of plural whiskers109 b including a crystalline semiconductor over the crystallinesemiconductor region 109 a. Note that the interface between thecrystalline semiconductor region 109 a and the whisker 109 b is unclear.A plane that is in the same level as the bottom of the deepest valley ofthe valleys formed among whiskers 109 b and is parallel to a surface ofthe electrode 103 is regarded as the interface between the crystallinesemiconductor region 109 a and the whisker 109 b.

The crystalline semiconductor region 109 a covers thefirst-conductivity-type crystalline semiconductor region 107. Inaddition, the whisker 109 b is a whisker-like protrusion, and aplurality of the protrusions are dispersed. Note that the whisker 109 bmay have a column-like shape such as a cylinder or a prism, or aneedle-like shape such as a cone or a pyramid. The top of the whisker109 b may be rounded. The diameter of the whisker 109 b is greater thanor equal to 100 nm and less than or equal to 10 μm, preferably greaterthan or equal to 500 nm and less than or equal to 3 μm. Further, thelength along the axis of the whisker 109 b is greater than or equal to300 nm and less than or equal to 20 μm, preferably greater than or equalto 500 nm and less than or equal to 15 μm. The photoelectric conversiondevice in this embodiment includes one or more of the above whiskers.

Note that the length along the axis of the whisker 109 b is the distancebetween the top (or the center of the top surface) of the whisker 109 band the crystalline semiconductor region 109 a along the axis runningthrough the top (or the center of the top surface) of the whisker 109 b.The thickness of the intrinsic crystalline semiconductor region 109 isthe sum of the thickness of the crystalline semiconductor region 109 aand the length of a line running from the top of the whisker 109 bperpendicularly to the crystalline semiconductor region 109 a (i.e., theheight of the whisker). The diameter of the whisker 109 b refers to alength of a longer axis of a transverse cross-sectional shape at theinterface between the crystalline semiconductor region 109 a and thewhisker 109 b.

Note that the direction in which the whisker 109 b extends from thecrystalline semiconductor region 109 a is referred to as a longitudinaldirection. A cross-sectional shape along the longitudinal direction isreferred to as a longitudinal cross-sectional shape. The shape of theplane normal to the longitudinal direction is referred to as atransverse cross-sectional shape.

In FIG. 1, the longitudinal directions of the whiskers 109 b included inthe intrinsic crystalline semiconductor region 109 extend in onedirection, for example, the direction normal to the surface of theelectrode 103. Note that the longitudinal direction of the whisker 109 bmay be substantially the same as the direction normal to the surface ofthe electrode 103. In that case, it is preferable that the differencebetween the angles of the two directions be typically within 5°.

Note that although the longitudinal directions of the whiskers 109 bincluded in the intrinsic crystalline semiconductor region 109 extend inone direction, for example, the direction normal to the surface of theelectrode 103 in FIG. 1, the longitudinal directions of the whiskers maybe varied. Typically, the intrinsic crystalline semiconductor region 109may include a whisker whose longitudinal direction is substantially thesame as the direction normal to the surface of the electrode 103 and awhisker whose longitudinal direction is different from the directionnormal to the surface of the electrode 103.

The second-conductivity-type crystalline semiconductor region 111 isformed using an re-type crystalline semiconductor. Note thatsemiconductor materials which can be used for thesecond-conductivity-type crystalline semiconductor region 111 aresimilar to those for the first-conductivity-type crystallinesemiconductor region 107.

In this embodiment, an interface between the intrinsic crystallinesemiconductor region 109 and the second-conductivity-type crystallinesemiconductor region 111 and a surface of the second-conductivity-typecrystalline semiconductor region 111 are uneven. Therefore, reflectanceof light incident on the insulating layer 113 can be reduced. Further,the light incident on the photoelectric conversion layer 106 isefficiently absorbed by the photoelectric conversion layer 106 owing toa light-trapping effect; thus, the characteristics of the photoelectricconversion device can be improved.

Note that a concentration gradient of an impurity element imparting thefirst conductivity type is preferably formed from thefirst-conductivity-type crystalline semiconductor region 107 toward thecrystalline semiconductor region 109, which are illustrated in FIG. 1.In other words, a concentration gradient of the impurity element ispreferably formed between the crystalline semiconductor region 107 andthe crystalline semiconductor region 109 (the region is also referred toas a contact portion). Note that since the interface between thecrystalline semiconductor region 107 and the crystalline semiconductorregion 109 is not clear, an embodiment of the present invention includesthe case where the concentration gradient is in the crystallinesemiconductor region 107, the case where the concentration gradient isin the crystalline semiconductor region 109, the case where theconcentration gradient is in the both regions, and the case where theconcentration gradient is in another region.

FIG. 6A is an enlarged view of a whisker in FIG. 1. As in FIG. 6A, partof the impurity element (X) imparting the first conductivity typeincluded in the crystalline semiconductor region 107 moves from thecrystalline semiconductor region 107 toward the crystallinesemiconductor region 109, whereby a concentration gradient illustratedin FIG. 6B is formed. In other words, the impurity element concentrationin the crystalline semiconductor region 107 is higher than that in thecrystalline semiconductor region 109.

By thus forming the concentration gradient of the impurity element fromthe crystalline semiconductor region 107 toward the crystallinesemiconductor region 109, a decrease in short-circuit current in thephotoelectric conversion device can be prevented. In other words, evenif the lifetime of minor carriers is shortened because of defects in thecrystalline semiconductor region including a group of whiskers, ashort-circuit current can be prevented from decreasing.

Note that the concentration gradient of the impurity element is notlimited to a continuous change of the concentration. For example, aregion whose concentration is higher than that of the crystallinesemiconductor region 109 and lower than that of the crystallinesemiconductor region 107 may be provided between the crystallinesemiconductor region 107 and the crystalline semiconductor region 109.

Note that an interface between a first-conductivity-type crystallinesemiconductor region 108 and the intrinsic crystalline semiconductorregion 109 may be uneven as illustrated in FIG. 2, whereas the interfacebetween the first-conductivity-type crystalline semiconductor region 107and the intrinsic crystalline semiconductor region 109 is flat inFIG. 1. The first-conductivity-type crystalline semiconductor region 108illustrated in FIG. 2 includes a crystalline semiconductor region 108 aincluding an impurity element imparting the first conductivity type anda group of plural whiskers 108 b including a crystalline semiconductorincluding the impurity element imparting the first conductivity typeover the crystalline semiconductor region 108 a. Note that the interfacebetween the crystalline semiconductor region 108 a and the whisker 108 bis unclear. A plane that is in the same level as the bottom of thedeepest valley of the valleys formed among whiskers 108 b and isparallel to a surface of the electrode 103 is regarded as the interfacebetween the crystalline semiconductor region 108 a and the whisker 108b.

The whisker 108 b is a whisker-like protrusion, and a plurality of theprotrusions are dispersed. Note that the whisker 108 b may have acolumn-like shape such as a cylinder or a prism, or a needle-like shapesuch as a cone or a pyramid. The top of the whisker 108 b may berounded.

The longitudinal directions of the whiskers 108 b included in thefirst-conductivity-type crystalline semiconductor region 108 extend inone direction, for example, the direction normal to the surface of theelectrode 103. Note that the longitudinal direction of the whisker 108 bmay be substantially the same as the direction normal to the surface ofthe electrode 103. In that case, it is preferable that the differencebetween the angles of the two directions be typically within 5°.

Note that although the longitudinal directions of the whiskers 108 bincluded in the first-conductivity-type crystalline semiconductor region108 extend in one direction, for example, the direction normal to thesurface of the electrode 103 in FIG. 2, the longitudinal directions ofthe whiskers may be varied. Typically, the first-conductivity-typecrystalline semiconductor region 108 may include a whisker whoselongitudinal direction is substantially the same as the direction normalto the surface of the electrode 103 and a whisker whose longitudinaldirection is different from the direction normal to the surface of theelectrode 103.

In the photoelectric conversion device illustrated in FIG. 2, theinterface between the first-conductivity-type crystalline semiconductorregion 108 and the intrinsic crystalline semiconductor region 109, theinterface between the intrinsic crystalline semiconductor region 109 andthe second-conductivity-type crystalline semiconductor region 111, andthe surface of the second-conductivity-type crystalline semiconductorregion 111 are uneven. Therefore, the reflectance of light incident onthe insulating layer 113 can be reduced. In addition, light incident onthe photoelectric conversion layer is efficiently absorbed by thephotoelectric conversion layer owing to a light-trapping effect.Accordingly, the characteristics of the photoelectric conversion devicecan be improved.

Note that a concentration gradient of an impurity element imparting thefirst conductivity type is preferably formed from thefirst-conductivity-type crystalline semiconductor region 108 toward thecrystalline semiconductor region 109, which are illustrated in FIG. 2.In other words, a concentration gradient of the impurity element ispreferably formed between the crystalline semiconductor region 108 andthe crystalline semiconductor region 109 (the region is also referred toas a contact portion). Note that since the interface between thecrystalline semiconductor region 108 and the crystalline semiconductorregion 109 is not clear, an embodiment of the present invention includesthe case where the concentration gradient is in the crystallinesemiconductor region 108, the case where the concentration gradient isin the crystalline semiconductor region 109, the case where theconcentration gradient is in the both regions, and the case where theconcentration gradient is in another region.

FIG. 7A is an enlarged view of a whisker in FIG. 2. As in FIG. 7A, partof the impurity element (X) imparting the first conductivity typeincluded in the crystalline semiconductor region 108 moves from thecrystalline semiconductor region 108 toward the crystallinesemiconductor region 109, whereby a concentration gradient illustratedin FIG. 7B is formed. In other words, the impurity element concentrationin the crystalline semiconductor region 108 is higher than that in thecrystalline semiconductor region 109.

By thus forming the concentration gradient of the impurity element fromthe crystalline semiconductor region 108 toward the crystallinesemiconductor region 109, a decrease in short-circuit current in thephotoelectric conversion device can be prevented. In other words, evenif the lifetime of minor carriers is shortened because of defects in thecrystalline semiconductor region including a group of whiskers, ashort-circuit current can be prevented from decreasing.

Note that the concentration gradient of the impurity element is notlimited to a continuous change of the concentration. For example, aregion whose concentration is higher than that of the crystallinesemiconductor region 109 and lower than that of the crystallinesemiconductor region 108 may be provided between the crystallinesemiconductor region 108 and the crystalline semiconductor region 109.

Note that the insulating layer 113 which has an anti-reflection functionis preferably formed over exposed surfaces of the electrode 103 and thesecond-conductivity-type crystalline semiconductor region 111.

For the insulating layer 113, a material whose refractive index isbetween the refractive indices of a light incident plane of thesecond-conductivity-type crystalline semiconductor region 111 and air isused. In addition, a material which transmits light with a predeterminedwavelength is used so that incidence of light on thesecond-conductivity-type crystalline semiconductor region 111 is notinterrupted. The use of such a material can prevent reflection at thelight incidence plane of the second-conductivity-type crystallinesemiconductor region 111. Note that as such a material, silicon nitride,silicon nitride oxide, or magnesium fluoride can be given, for example.

Note that as illustrated in FIG. 3, a grid electrode 115 for reducingthe resistance of the second-conductivity-type crystalline semiconductorregion 111 may be provided on the second-conductivity-type crystallinesemiconductor region 111.

The grid electrode 115 is formed with a layer including a metal elementsuch as silver, copper, aluminum, or palladium. By providing the gridelectrode 115 to be in contact with the second-conductivity-typecrystalline semiconductor region 111, resistance loss of thesecond-conductivity-type crystalline semiconductor region 111 can bereduced and electrical characteristics can be improved, in particular,under high illuminance.

Although not illustrated, an electrode may be provided over thesecond-conductivity-type crystalline semiconductor region 111. Theelectrode is formed using a light-transmitting conductive layer of analloy of indium oxide and tin oxide (ITO), zinc oxide (ZnO), tin oxide(SnO₂), zinc oxide containing aluminum, or the like.

Next, a method for manufacturing the photoelectric conversion deviceillustrated in FIG. 1 will be described with reference to FIGS. 4A to4C.

As in FIG. 4A, the conductive layer 104 is formed over the substrate101. The conductive layer 104 can be formed by a printing method, asol-gel method, a coating method, an ink-jet method, a CVD method, asputtering method, an evaporation method, or the like, as appropriate.Note that in the case where the conductive layer 104 has a foil shape,it is not necessary to provide the substrate 101. Further, roll-to-rollprocessing can be employed.

Next, as in FIG. 4B, the first-conductivity-type crystallinesemiconductor region 107, the intrinsic crystalline semiconductor region109, and the second-conductivity-type crystalline semiconductor region111 are formed by an LPCVD method. The LPCVD method is performed asfollows: heating is performed at a temperature higher than 550° C. andequal to or lower than the temperature at which an LPCVD apparatus andthe conductive layer 104 can withstand, preferably higher than or equalto 580° C. and lower than 650° C.; at the same time, at least adeposition gas containing silicon is used as a source gas; and thepressure in a reaction chamber of the LPCVD apparatus is set to apressure higher than or equal to a lower limit at which the pressure canbe maintained while the source gas flows and lower than or equal to 200Pa. Examples of the deposition gas containing silicon include siliconhydride, silicon fluoride, and silicon chloride; typically, SiH₄, Si₂H₆,SiF₄, SiCl₄, Si₂Cl₆, and the like are given. Note that hydrogen may beintroduced into the source gas.

When the first-conductivity-type crystalline semiconductor region 107 isformed by an LPCVD method, the mixed layer 105 may be formed between theconductive layer 104 and the first-conductivity-type crystallinesemiconductor region 107 depending on heating conditions. In a step offorming the first-conductivity-type crystalline semiconductor region107, active species of the source gas are constantly supplied to aportion being deposited, and silicon diffuses from thefirst-conductivity-type crystalline semiconductor region 107 to theconductive layer 104, whereby the mixed layer 105 is formed. For thisreason, a low-density region (a sparse region) is not easily formed atan interface between the conductive layer 104 and thefirst-conductivity-type crystalline semiconductor region 107, and thusthe characteristics of the interface between the conductive layer 104and the first-conductivity-type crystalline semiconductor region 107 areimproved, whereby series resistance can be reduced.

The first-conductivity-type crystalline semiconductor region 107 isformed by an LPCVD method in which diborane and a deposition gascontaining silicon are introduced into a reaction chamber of an LPCVDapparatus as a source gas. The thickness of the first-conductivity-typecrystalline semiconductor region 107 is greater than or equal to 5 nmand less than or equal to 500 nm. Here, a crystalline silicon layer towhich boron is added is formed as the first-conductivity-typecrystalline semiconductor region 107.

Then, the introduction of diborane into the reaction chamber of theLPCVD apparatus is stopped. Then, the intrinsic crystallinesemiconductor region 109 is formed by an LPCVD method in which adeposition gas containing silicon is introduced as a source gas into thereaction chamber of the LPCVD apparatus. The thickness of the intrinsiccrystalline semiconductor region 109 is greater than or equal to 500 nmand less than or equal to 20 μm. Here, a crystalline silicon layer isformed as the intrinsic crystalline semiconductor region 109. In thisstep, part of boron (X) included in the crystalline semiconductor region107 moves from the crystalline semiconductor region 107 toward thecrystalline semiconductor region 109 as in FIG. 6A, whereby aconcentration gradient of boron is formed as in FIG. 6B.

Then, the second-conductivity-type crystalline semiconductor region 111is formed by an LPCVD method in which phosphine or arsine and adeposition gas containing silicon are introduced as a source gas intothe reaction chamber of the LPCVD apparatus. The thickness of thesecond-conductivity-type crystalline semiconductor region 111 is greaterthan or equal to 5 nm and less than or equal to 500 nm. Here, acrystalline silicon layer to which phosphorus or arsenic is added isformed as the second-conductivity-type crystalline semiconductor region111.

Through the above steps, the photoelectric conversion layer 106including the first-conductivity-type crystalline semiconductor region107, the intrinsic crystalline semiconductor region 109, and thesecond-conductivity-type crystalline semiconductor region 111 can beformed.

Note that, in the manufacturing process of the photoelectric conversiondevice illustrated in FIG. 1, in the case where the introduction ofdiborane into the reaction chamber of the LPCVD apparatus is stoppedbefore whiskers are formed in the first-conductivity-type crystallinesemiconductor region 107, the interface between thefirst-conductivity-type crystalline semiconductor region 107 and theintrinsic crystalline semiconductor region is flat as in FIG. 1. On theother hand, in the case where the introduction of diborane into thereaction chamber of the LPCVD apparatus is stopped after whiskers areformed in the first-conductivity-type crystalline semiconductor region,the interface between the first-conductivity-type crystallinesemiconductor region 108 and the intrinsic crystalline semiconductorregion 109 is uneven as in FIG. 2. In the step of forming the intrinsiccrystalline semiconductor region 109, part of boron (X) included in thecrystalline semiconductor region 108 moves from the crystallinesemiconductor region 108 toward the crystalline semiconductor region 109as in FIG. 7A, whereby a concentration gradient of boron is formed as inFIG. 7B.

A surface of the conductive layer 104 may be cleaned with hydrofluoricacid before the formation of the first-conductivity-type crystallinesemiconductor region 107. This step can enhance the adhesiveness betweenthe electrode 103 and the first-conductivity-type crystallinesemiconductor region 107.

Further, nitrogen or a rare gas such as helium, neon, argon, or xenonmay be mixed into the source gas of the first-conductivity-typecrystalline semiconductor region 107, intrinsic crystallinesemiconductor region 109, and second-conductivity-type crystallinesemiconductor region 111. In the case where nitrogen or a rare gas isadded to the source gas of the first-conductivity-type crystallinesemiconductor region 107, intrinsic crystalline semiconductor region109, and second-conductivity-type crystalline semiconductor region 111,the density of whiskers can be increased.

After the formation of one or more of the first-conductivity-typecrystalline semiconductor region 107, the intrinsic crystallinesemiconductor region 109, and the second-conductivity-type crystallinesemiconductor region 111, in the case where the introduction of thesource gas into the reaction chamber of the LPCVD apparatus is stoppedand the temperature is maintained in a vacuum state (i.e., vacuumheating is performed), the density of whiskers included in thefirst-conductivity-type crystalline semiconductor region 107 or theintrinsic crystalline semiconductor region 109 can be increased.

Then, as in FIG. 4C, the insulating layer 113 is formed over thesecond-conductivity-type crystalline semiconductor region 111. Theinsulating layer 113 can be formed by a CVD method, a sputtering method,an evaporation method, or the like.

With the above steps, a photoelectric conversion device with a highconversion efficiency can be manufactured even when the siliconsubstrate is not etched to form an electrode having an uneven structure(also referred to as a textured structure).

Embodiment 2

In this embodiment, a method for manufacturing a photoelectricconversion layer which has fewer defects than the photoelectricconversion layer in Embodiment 1 is described.

After one or more of the first-conductivity-type crystallinesemiconductor region 107, the first-conductivity-type crystallinesemiconductor region 108, the intrinsic crystalline semiconductor region109, and the second-conductivity-type crystalline semiconductor region111, which are described in Embodiment 1, are formed, the temperature ofa reaction chamber in an LPCVD apparatus is set at a temperature higherthan or equal to 400° C. and lower than or equal to 450° C., theintroduction of a source gas into the LPCVD apparatus is stopped, andhydrogen is introduced. Then, in a hydrogen atmosphere, heat treatmentat a temperature higher than or equal to 400° C. and lower than or equalto 450° C. is performed. In this manner, dangling bonds in one or moreof the first-conductivity-type crystalline semiconductor region 107, thefirst-conductivity-type crystalline semiconductor region 108, theintrinsic crystalline semiconductor region 109, and thesecond-conductivity-type crystalline semiconductor region 111 can beterminated with hydrogen. The heat treatment is also referred to as ahydrogenation treatment. As a result of the heat treatment, defects inone or more of the first-conductivity-type crystalline semiconductorregion 107, the first-conductivity-type crystalline semiconductor region108, the intrinsic crystalline semiconductor region 109, and thesecond-conductivity-type crystalline semiconductor region 111 can bereduced, which leads to less recombination of photoexcited carriers indefects and also leads to an increase in conversion efficiency of thephotoelectric conversion device.

Note that the hydrogenation treatment is preferably performed at leastafter the intrinsic crystalline semiconductor region 109 is formed. Inthat case, the conversion efficiency of the photoelectric conversiondevice can be increased while the throughput is increased.

Embodiment 3

In this embodiment, the structure of a so-called tandem photoelectricconversion device in which a plurality of photoelectric conversionlayers are stacked is described with reference to FIG. 5. Although twophotoelectric conversion layers are stacked in this embodiment, three ormore photoelectric conversion layers may be stacked. In the followingdescription, the photoelectric conversion layer which is closest to thelight incident surface may be referred to as a top cell and thephotoelectric conversion layer which is farthest from the light incidentsurface may be referred to as a bottom cell.

FIG. 5 illustrates a photoelectric conversion device in which thesubstrate 101, the electrode 103, the photoelectric conversion layer 106which is the bottom cell, a photoelectric conversion layer 120 which isthe top cell, and the insulating layer 113 are stacked. Here, thephotoelectric conversion layer 106 includes the first-conductivity-typecrystalline semiconductor region 107, the intrinsic crystallinesemiconductor region 109, and the second-conductivity-type crystallinesemiconductor region 111, which are described in Embodiment 1. Thephotoelectric conversion layer 120 includes a third-conductivity-typesemiconductor region 121, an intrinsic semiconductor region 123, and afourth-conductivity-type semiconductor region 125. The band gap of theintrinsic crystalline semiconductor region 109 in the photoelectricconversion layer 106 is preferably different from that of the intrinsicsemiconductor region 123 in the photoelectric conversion layer 120. Useof semiconductors having different band gaps makes it possible to absorba wide wavelength range of light; thus, a photoelectric conversionefficiency can be improved.

For example, a semiconductor with a large band gap can be used for thetop cell while a semiconductor with a small band gap can be used for thebottom cell, and needless to say, vice versa. Here, as an example, astructure where a crystalline semiconductor (typically, crystallinesilicon) is used in the photoelectric conversion layer 106, which is thebottom cell, and an amorphous semiconductor (typically, amorphoussilicon) is used in the photoelectric conversion layer 120, which is thetop cell, is described.

Note that although a structure where light is incident on the insulatinglayer 113 is described in this embodiment, one embodiment of thedisclosed invention is not limited thereto. Light may be incident on therear surface of the substrate 101 (the lower surface in the drawing).

The structures of the substrate 101, the electrode 103, thephotoelectric conversion layer 106, and the insulating layer 113 aresimilar to those in the above embodiments and description thereof isomitted here.

In the photoelectric conversion layer 120, which is the top cell, asemiconductor layer including a semiconductor material to which animpurity element imparting a conductivity type is added is typicallyused as the third-conductivity-type semiconductor region 121 and thefourth-conductivity-type semiconductor region 125. Details of thesemiconductor material and the like are similar to those of thefirst-conductivity-type crystalline semiconductor region 107 inEmbodiment 1. In this embodiment, the case where silicon is used as thesemiconductor material, the third conductivity type is p-type, and thefourth conductivity type is n-type is described. In addition, thecrystallinity of the semiconductor layer is amorphous. It is needless tosay that the third conductivity type may be n-type, the fourthconductivity type may be p-type, and the semiconductor layer is notnecessarily amorphous.

For the intrinsic semiconductor region 123, silicon, silicon carbide,germanium, gallium arsenide, indium phosphide, zinc selenide, galliumnitride, silicon germanium, or the like is used. Alternatively, asemiconductor material including an organic material, a metal oxidesemiconductor material, or the like can be used.

In this embodiment, amorphous silicon is used for the intrinsicsemiconductor region 123. It is needless to say that the intrinsicsemiconductor region 123 may be formed using a semiconductor materialwhich is not silicon and has a band gap different from that of theintrinsic crystalline semiconductor region 109 in the bottom cell. Here,the thickness of the intrinsic semiconductor region 123 is preferablysmaller than that of the intrinsic crystalline semiconductor region 109and is typically greater than or equal to 50 nm and less than or equalto 1000 nm, preferably greater than or equal to 100 nm and less than orequal to 450 nm.

A plasma CVD method, an LPCVD method, or the like may be employed forforming the third-conductivity-type semiconductor region 121, theintrinsic semiconductor region 123, and the fourth-conductivity-typesemiconductor region 125. In the case of a plasma CVD method, theintrinsic semiconductor region 123 can be formed in such a manner thatthe pressure in a reaction chamber of a plasma CVD apparatus istypically greater than or equal to 10 Pa and less than or equal to 1332Pa, hydrogen and a deposition gas containing silicon are introduced as asource gas to the reaction chamber, and high-frequency electric power issupplied to an electrode to cause glow discharge. Thethird-conductivity-type semiconductor region 121 can be formed using theabove source gas to which diborane is added. The third-conductivity-typesemiconductor region 121 is formed with a thickness of greater than orequal to 1 nm and less than or equal to 100 nm, preferably greater thanor equal to 5 nm and less than or equal to 50 nm. Thefourth-conductivity-type semiconductor region 125 can be formed usingthe above source gas to which phosphine or arsine is added. Thefourth-conductivity-type semiconductor region 125 is formed with athickness of greater than or equal to 1 nm and less than or equal to 100nm, preferably greater than or equal to 5 nm and less than or equal to50 nm.

Alternatively, the third-conductivity-type semiconductor region 121 maybe formed by forming an amorphous silicon layer by a plasma CVD methodor an LPCVD method without adding an impurity element imparting aconductivity type and then adding boron by a method such as ioninjection. The fourth-conductivity-type semiconductor region 125 may beformed by forming an amorphous silicon layer by a plasma CVD method oran LPCVD method without adding an impurity element imparting aconductivity type and then adding phosphorus or arsenic by a method suchas ion injection.

As described above, by using amorphous silicon for the photoelectricconversion layer 120, light having a wavelength of less than 800 nm canbe effectively absorbed and subjected to photoelectric conversion.Further, by using crystalline silicon for the photoelectric conversionlayer 106, light having a longer wavelength (e.g., a wavelength up toapproximately 1200 nm) can be absorbed and subjected to photoelectricconversion. Such a structure (a so-called tandem structure) in whichphotoelectric conversion layers having different band gaps are stackedcan significantly increase a photoelectric conversion efficiency.

Note that although amorphous silicon having a large band gap is used inthe top cell and crystalline silicon having a small band gap is used inthe bottom cell in this embodiment, one embodiment of the disclosedinvention is not limited thereto. The semiconductor materials havingdifferent band gaps can be used in appropriate combination to form thetop cell and the bottom cell. The structure of the top cell and thestructure of the bottom cell can be replaced with each other to form thephotoelectric conversion device. Alternatively, a stacked layerstructure in which three or more photoelectric conversion layers arestacked can be employed.

With the above structure, the conversion efficiency of a photoelectricconversion device can be increased.

Embodiment 4

In this embodiment, an example where a conductive layer is formed by awet process over a second-conductivity-type crystalline semiconductorregion in a photoelectric conversion device is described with referenceto FIG. 8.

FIG. 8 is a photoelectric conversion device including the substrate 101,the first electrode 103, the first-conductivity-type crystallinesemiconductor region 107, the intrinsic crystalline semiconductor region109, the second-conductivity-type crystalline semiconductor region 111,and a conductive layer 213. The second conductivity type is opposite tothe first conductivity type. The first-conductivity-type crystallinesemiconductor region 107, the intrinsic crystalline semiconductor region109, and the second-conductivity-type crystalline semiconductor region111 function as the photoelectric conversion layer 106.

The first electrode 103 may include the conductive layer 104 and themixed layer 105. In addition, an interface between the first electrode103 and the first-conductivity-type crystalline semiconductor region 107is flat. The intrinsic crystalline semiconductor region 109 includes aplurality of whiskers (a group of whiskers). Accordingly, the interfacebetween the intrinsic crystalline semiconductor region 109 and thesecond-conductivity-type crystalline semiconductor region 111, and asurface of the second-conductivity-type crystalline semiconductor region111 are uneven.

In this embodiment, the conductive layer 213 is formed by a wet processover part of or the whole of the second-conductivity-type crystallinesemiconductor region 111. Thus, the conductive layer 213 can be formedwith a good coverage over a surface of the second-conductivity-typecrystalline semiconductor region 111 which has an uneven surface owingto the formation of whiskers. By forming the conductive layer 213 by awet process over the surface of the second-conductivity-type crystallinesemiconductor region 111 which has an uneven surface owing to theformation of whiskers, the resistance of the light incident surface canbe reduced. Further, the conductive layer 213 may be used as anelectrode. A material which is used for the conductive layer 213 ispreferably a material which transmits light in a wavelength region whichcan be absorbed by a semiconductor region serving as the photoelectricconversion layer 106.

A wet process can be a coating method such as a dip coating method, aspin coating method, a spray coating method, an ink jetting method, or aprinting method. Alternatively, an electrolytic plating method, anelectroless plating method, or the like can be used.

A coating liquid used in a coating method may be a liquid or aliquid-like substance such as a sol or a gel which contains a conductivematerial. The conductive material may be a fine particle of alight-transmitting conductive metal oxide such as indium oxide-tin oxidealloy (ITO), zinc oxide (ZnO), tin oxide (SnO₂), or zinc oxidecontaining aluminum; a fine particle of a metal such as gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), or silver(Ag); or a conductive polymer such as conductive polyaniline, conductivepolypyrrole, conductive polythiophene, polyethylenedioxythiophene(PEDOT), or polystyrene sulfonate (PSS). In the case where a fineparticle is used as the conductive material, the surface of the fineparticle may be coated with an organic substance or the like in order toimprove dispersibility. A solvent (or a disperse medium) of the liquidwhich contains a conductive material can be water, alcohols,hydrocarbon-based compounds, ether compounds, or the like. Thesesolvents (or disperse mediums) may be used alone or two or more of thesesolvents may be used in combination.

When a coating method is used as a wet process, a liquid or liquid-likesubstance which contains a conductive material is applied, dried, andbaked, whereby the conductive layer 213 can be formed. When a coatingmethod is used as a wet process, the thickness of the conductive layer213 can be easily increased and thus the resistance of the conductivelayer 213 can be reduced.

When the conductive layer 213 is thick, the surface of the conductivelayer 213 becomes flat. In this case, the surface of the conductivelayer 213 may be processed to be uneven so that reflectance of incidentlight may be reduced and that the characteristics of the photoelectricconversion device may be improved owing to a light-trapping effect.

Before the conductive layer 213 is formed, another conductive layer (notshown) may be formed over part of or the whole of thesecond-conductivity-type crystalline semiconductor region 111. Forexample, before the conductive layer 213 is formed, a conductive layermay be formed of a light-transmitting conductive material such as anindium oxide-tin oxide alloy (ITO), zinc oxide (ZnO), tin oxide (SnO₂),or zinc oxide containing aluminum by a dry process such as a CVD method,a sputtering method, or an evaporation method. By thus providing such aconductive layer in advance, the surface of the second-conductivity-typecrystalline semiconductor region 111 can be protected. In addition, bythus providing such a conductive layer in advance, the adhesion betweenthe conductive layer 213 and the second-conductivity-type crystallinesemiconductor region 111 can be improved.

Alternatively, the conductive layer 213 may be a conductive liquid (aliquid containing an electrolyte) which is provided over thesecond-conductivity-type crystalline semiconductor region 111 to fill aspace between whiskers and is used as an electrode. In this case, theconductive liquid can be introduced into a space between the substrate101 and a second substrate which faces the substrate 101, and thensealed with a sealant to form the conductive layer 213. In any case, byproviding an electrode to fill a space between whiskers, the resistanceof the light incident surface can be reduced.

This embodiment can be combined with any of the other embodiments asappropriate.

Example

In this example, difference in regular reflectance between a sample thatis a titanium foil and a sample including a titanium foil and a group ofwhiskers formed of polysilicon on the titanium foil is described.

First, a method for forming the samples is described.

<Sample 1>

As the sample 1, a titanium foil with a thickness of 0.1 mm which wascut in a circle with a diameter φ of 12 mm was used.

<Sample 2>

A polysilicon layer including a group of whiskers was formed by an LPCVDmethod on a titanium foil with a shape similar to that of the sample 1,i.e. a thickness of 0.1 mm and a diameter φ of 12 mm. The polysiliconlayer was formed by introducing silane for deposition for 2 hours and 15minutes at a flow rate of 300 sccm into a process chamber in which thepressure was set to 13 Pa and the substrate temperature was set to 600°C.

FIG. 9 illustrates the measurement results of the regular reflectance ofthe samples 1 and 2 with a spectrophotometer (U-4100 Spectrophotometer,manufactured by Hitachi High-Technologies Corporation). Here, thesamples 1 and 2 were irradiated with light having wavelengths from 200nm to 1200 nm with a sampling interval of 2 nm. An incident angle of thelight incident on the samples was 5° and the reflectance of the light(i.e., 5-degree regular reflectance) was measured. The reflectance ofthe sample 1 is shown by a broken line 501 and the reflectance of thesample 2 is shown by a solid line 502. The horizontal axis representsthe wavelength of the irradiation light and the vertical axis representsthe reflectance of the irradiation light.

According to FIG. 9, the light reflectance of the sample 2 in which thepolysilicon layer including the group of whiskers is formed on thesurface of the titanium foil is extremely low, 0.14% at most, whichmeans that there is almost no reflection of light. Note that since thesignal-to-noise ratio (SN ratio) is small in a wavelength range of 850nm to 894 nm, the reflectance is negative. In contrast, the sample 1that is the titanium foil has a regular reflectance of 2% to 15%. Theabove results show that the reflectance can be reduced by forming thepolysilicon layer including the group of whiskers on the surface of thetitanium foil.

EXPLANATION OF REFERENCE

101: substrate, 103: electrode, 104: conductive layer, 105: mixed layer,106: photoelectric conversion layer, 107: crystalline semiconductorregion, 108: crystalline semiconductor region, 108 a: crystallinesemiconductor region, 108 b: whisker, 109: crystalline semiconductorregion, 109 a: crystalline semiconductor region, 109 b: whisker, 111:crystalline semiconductor region, 113: insulating layer, 115: gridelectrode, 120: photoelectric conversion layer, 121: semiconductorregion, 123: semiconductor region, 125: semiconductor region, 213:conductive layer, 501: broken line, 502: solid line.

This application is based on Japanese Patent Application serial no.2010-139993 filed with Japan Patent Office on Jun. 18, 2010, the entirecontents of which are hereby incorporated by reference.

1. A photoelectric conversion device comprising: a first crystallinesemiconductor region; a second crystalline semiconductor region over thefirst crystalline semiconductor region; and a third crystallinesemiconductor region over the second crystalline semiconductor region,wherein the first crystalline semiconductor region includes a firstimpurity element of a first conductivity type, wherein the secondcrystalline semiconductor region comprises a plurality of whiskers,wherein a surface of the third crystalline semiconductor region isuneven, and wherein a concentration gradient of the first impurityelement of the first conductivity type is formed from the firstcrystalline semiconductor region toward the second crystallinesemiconductor region.
 2. The photoelectric conversion device accordingto claim 1, wherein an interface between the second crystallinesemiconductor region and the third crystalline semiconductor region isuneven.
 3. The photoelectric conversion device according to claim 1,wherein the third crystalline semiconductor region includes a secondimpurity element of a second conductivity type, wherein the firstconductivity type is one of p-type and n-type, and wherein the secondconductivity type is the other of the p-type and the n-type.
 4. Thephotoelectric conversion device according to claim 1, wherein theconcentration gradient of the first impurity element of the firstconductivity type is continuous change, and wherein the first impurityelement is boron.
 5. The photoelectric conversion device according toclaim 1, wherein longitudinal directions of the plurality of whiskersare varied.
 6. The photoelectric conversion device according to claim 1,wherein the first crystalline semiconductor region is formed over anelectrode, and wherein longitudinal direction of the plurality ofwhiskers are substantially the same as the direction normal to a surfaceof the electrode.
 7. A photoelectric conversion device comprising: afirst crystalline semiconductor region; a second crystallinesemiconductor region over the first crystalline semiconductor region;and a third crystalline semiconductor region over the second crystallinesemiconductor region, wherein the first crystalline semiconductor regionincludes a first impurity element of a first conductivity type, whereinthe first crystalline semiconductor region comprises a plurality ofwhiskers, wherein a surface of the third crystalline semiconductorregion is uneven, and wherein a concentration gradient of the firstimpurity element of the first conductivity type is formed from the firstcrystalline semiconductor region toward the second crystallinesemiconductor region.
 8. The photoelectric conversion device accordingto claim 7, wherein an interface between the first crystallinesemiconductor region and the second crystalline semiconductor region isuneven.
 9. The photoelectric conversion device according to claim 7,wherein the third crystalline semiconductor region includes a secondimpurity element of a second conductivity type, wherein the firstconductivity type is one of p-type and n-type, and wherein the secondconductivity type is the other of the p-type and the n-type.
 10. Thephotoelectric conversion device according to claim 7, wherein theconcentration gradient of the first impurity element of the firstconductivity type is continuous change, and wherein the first impurityelement is boron.
 11. The photoelectric conversion device according toclaim 7, wherein longitudinal directions of the plurality of whiskersare varied.
 12. The photoelectric conversion device according to claim7, wherein the first crystalline semiconductor region is formed over anelectrode, and wherein longitudinal directions of the plurality ofwhiskers are substantially the same as the direction normal to a surfaceof the electrode.
 13. A photoelectric conversion device comprising: afirst crystalline semiconductor region; a second crystallinesemiconductor region over the first crystalline semiconductor region; athird crystalline semiconductor region over the second crystallinesemiconductor region; a first semiconductor region over the thirdcrystalline semiconductor region; a second semiconductor region over thefirst semiconductor region; and a third semiconductor region over thesecond semiconductor region, wherein the first crystalline semiconductorregion includes a first impurity element of a first conductivity type,wherein the second crystalline semiconductor region comprises aplurality of whiskers, and wherein a concentration gradient of the firstimpurity element of the first conductivity type is formed from the firstcrystalline semiconductor region toward the second crystallinesemiconductor region.
 14. The photoelectric conversion device accordingto claim 13, wherein an interface between the second crystallinesemiconductor region and the third crystalline semiconductor region isuneven.
 15. The photoelectric conversion device according to claim 13,wherein the third crystalline semiconductor region includes a secondimpurity element of a second conductivity type, wherein the firstconductivity type is one of p-type and n-type, and wherein the secondconductivity type is the other of the p-type and the n-type.
 16. Thephotoelectric conversion device according to claim 15, wherein the firstsemiconductor region includes a third impurity element of a thirdconductivity type, wherein the third semiconductor region includes afourth impurity element of a fourth conductivity type, wherein theconductivity type of the third conductivity type and the firstconductivity type are the same, and wherein the conductivity type of thefourth conductivity type and the second conductivity type are the same.17. The photoelectric conversion device according to claim 13, wherein aband gap of the second crystalline semiconductor region is differentfrom a band gap of the second semiconductor region.
 18. Thephotoelectric conversion device according to claim 13, wherein theconcentration gradient of the first impurity element of the firstconductivity type is continuous change, and wherein the first impurityelement is boron.
 19. The photoelectric conversion device according toclaim 13, wherein longitudinal directions of the plurality of whiskersare varied.
 20. The photoelectric conversion device according to claim13, wherein the first crystalline semiconductor region is formed over anelectrode, and wherein longitudinal directions of the plurality ofwhiskers are substantially the same as the direction normal to a surfaceof the electrode.
 21. A photoelectric conversion device comprising: afirst crystalline semiconductor region; a second crystallinesemiconductor region over the first crystalline semiconductor region; athird crystalline semiconductor region over the second crystallinesemiconductor region; a first semiconductor region over the thirdcrystalline semiconductor region; a second semiconductor region over thefirst semiconductor region; and a third semiconductor region over thesecond semiconductor region, wherein the first crystalline semiconductorregion includes a first impurity element of a first conductivity type,wherein the first crystalline semiconductor region comprises a pluralityof whiskers, and wherein a concentration gradient of the first impurityelement of the first conductivity type is formed from the firstcrystalline semiconductor region toward the second crystallinesemiconductor region.
 22. The photoelectric conversion device accordingto claim 21, wherein an interface between the first crystallinesemiconductor region and the second crystalline semiconductor region isuneven.
 23. The photoelectric conversion device according to claim 21,wherein the third crystalline semiconductor region includes a secondimpurity element of a second conductivity type, wherein the firstconductivity type is one of p-type and n-type, and wherein the secondconductivity type is the other of the p-type and the n-type.
 24. Thephotoelectric conversion device according to claim 23, wherein the firstsemiconductor region includes a third impurity element of a thirdconductivity type, wherein the third semiconductor region includes afourth impurity element of a fourth conductivity type, wherein theconductivity type of the third conductivity type and the firstconductivity type is the same, and wherein the conductivity type of thefourth conductivity type and the second conductivity type is the same.25. The photoelectric conversion device according to claim 21, wherein aband gap of the second crystalline semiconductor region is differentfrom a band gap of the second semiconductor region.
 26. Thephotoelectric conversion device according to claim 21, wherein theconcentration gradient of the first impurity element of the firstconductivity type is continuous change, and wherein the first impurityelement is boron.
 27. The photoelectric conversion device according toclaim 21, wherein longitudinal directions of the plurality of whiskersare varied.
 28. The photoelectric conversion device according to claim21, wherein the first crystalline semiconductor region is formed over anelectrode, and wherein longitudinal directions of the plurality ofwhiskers are substantially the same as the direction normal to a surfaceof the electrode.